As known in the art, a primary consideration in any digital communications system is the channel bandwidth and channel separation required to transmit information. Therefore, digital systems are typically designed to utilize channel bandwidth as efficiently as possible. For example, in systems utilizing frequency division multiplexing, maximum spectral efficiency is obtained by spacing frequency channels very close to one another in an available spectrum.
Minimum carrier spacing is limited in practice, however, by adjacent channel interference. Adjacent channel interference is defined as the interference resulting when carrier frequencies are spaced close enough to one another that information signals received from number of carriers overlap in the frequency spectrum, or in other words, the interferences created when a number of close by systems (base stations) use the same or adjacent frequency channels. In practice, the minimum allowable carrier spacing is a function of the bandwidths of the information signals, the practical limitations associated with receiver filtering, and the signal modulation and coding schemes used. Number of design improvement providing increased suppression of adjacent channel interference have been suggested to increase system capacity, relax coding and modulation design requirements, or improve signal quality.
In conventional systems, adjacent channel interference is suppressed in a number of ways. For example, demodulation parameters such as linear or decision feedback equalization filter coefficients are adapted to minimize noise and adjacent channel interference together. Alternatively, spectrally efficient continuous phase modulation (CPM) techniques can be used to reduce the effects of adjacent channel interference.
U.S. Pat. No. 6,108,517 discloses methods and apparatus for receiving adjacent channel signals wherein adjacent channel interference effects are minimized through joint demodulation of the adjacent channel signals. A channel associated with each signal and each corresponding frequency band is estimated and used to form joint branch metrics for joint sequence estimation. Thus, a baseband processor receives baseband samples corresponding to a certain carrier frequency, and then jointly demodulates at least two information streams corresponding to different carrier frequencies in dependence upon the received baseband samples.
US application 20020136169 discloses a wireless access system for allocating and synchronizing uplink and downlink of TDD frames. By this publication, a TDD frame transmission synchronization apparatus is used in a fixed wireless access network comprising a plurality of base stations capable of bi-directional time division duplex (TDD) communication with a plurality of wireless access subscriber devices. The TDD frame transmission synchronization apparatus comprises a frame allocation controller for receiving from a first radio frequency (RF) modem shelf associated with a first base station access requests generated by a first group of wireless access devices communicating with the first base station and determining from traffic requirements associated with the access requests a time duration of a longest downlink portion of TDD frames used by a first one of a plurality of RF modems in the RF modem shelf to communicate with a first wireless access subscriber device. The frame allocation controller further determines a frame allocation of the downlink portion and the upLink portion of TDD frames used by the plurality of RF modems to communicate with the first group of wireless access subscriber devices.
WO 02103921 describes a method for reducing interferences in a Time Division Duplexing (TDD) system. By this method the amount of interference to communications in TDD mode between a TDD mobile and a TDD base station from a mobile operating in Frequency Division Duplexing (FDD) mode located near the TDD base station can be reduced by passing information regarding the interfering FDD signal to the TDD base station, where it is processed and subtracted from the received TDD signal, which includes the interference, so as to obtain the pure TDD signal. Either the FDD data or an FDD signal regenerated from the FDD data or simply the received FDD signal can be passed to the TDD base station. The TDD base station can regenerate an FDD signal, if required, which is then filtered using TDD filters and normalized by correlating and scaling the filtered FDD signal to a received TDD signal. The normalized FDD signal is then subtracted from the received TDD signal to reduce the interference.
U.S. Pat. No. 5,710,797 describes a single transducer digital communication receiver which is capable of extracting the data bits of at least one desired signal in the presence of interfering signals of similar type, so as to provide a system in which overlapping transmissions are tolerated and allowed. This publication describes a specific frequency plan, which includes an appropriate digital demodulator that extracts the data bits of the desired signal(s) in the presence of closely spaced signals. The disclosure enables reduced channel spacing in digital communication systems and thereby increases the system capacity (i.e. the number of users per bandwidth unit) without incurring any significant loss in system performance (e.g. power margins, BER, and channel availability). It also allows a reduced power margin that is required to maintain a pre-specified performance level without sacrificing system capacity.
WO 9837646 describes a method of estimating interference level in a cellular radio system that comprises in each cell a base station and a number of subscriber terminals. The terminals measure power levels at those frequencies which are on the list that is maintained by the terminal and to which the terminal can perform a handover.
EP 719480 discloses a method for simulating interferences received by subscribers in a spread spectrum communications system, by which users from a plurality of cells communicate information signals between one another through at least one base station and using and code division multiple access (CDMA) spread spectrum type communication signals. The disclosed method includes determining a first composite signal energy associated with signal transmission from the base station transmitter over a first set of simulated channels. The simulation method further contemplates estimating a first average data rate for the first composite signal energy. Signal power transmitted over the one channel is adjusted in accordance with a first interference signal which is based on the first composite signal energy and first average data rate.
U.S. Pat. No. 6,442,387 teaches a method for estimating the interference level in a cellular radio system comprising in each cell a base station in each cell, and a number of subscriber terminals communicating with the base station. The terminals measure power levels from those frequencies which are on the list that is maintained by the terminal and comprised those base stations and frequencies to which the terminal can perform a handover. In the solution of the invention the list also comprises frequencies whose power level information is used for the frequency planning of the system. Frequency planning can thus be implemented without any separate means.
In our copending application, IL 160832, there is described a method used in a wireless network comprising at least one base station and a plurality of subscriber terminals, for allocating time slots for operating at frequencies that are affected by interferences caused by another base station or terminal(s) belonging to a different network, where the method comprises dividing a time slot allocated for transmission along an interfered channel that experiences interferences into time sub-frames, where at least one of the time sub-frames is reserved for transmitting radio signals from the base station to a terminal belonging to that network and experiencing interferences while at least another time sub-frame is reserved for transmitting radio signals in another network from another base station to a subscriber terminal associated therewith.
The disclosure of the references mentioned herein throughout the present specification are hereby incorporated by reference.
As noted above, however, minimizing or avoiding adjacent channel interference using the above described systems do not provide an adequate solutions for broad applications. In particularly, when there is a cell using IEEE 802.16 technology based on orthogonal Frequency Division Multiplexing (OFDM) modulation with capability of beam-forming while using Advanced Antenna System (AAS) at the Base Station (BS). Downlink (DL) transmissions of the cell may cause interference to DL transmissions at adjacent cells using the same technology and operating at same or even adjacent frequency channel.
Thus, there is a need for improved methods and apparatus for significantly reducing the impact of interferences in cases as described above.